Electrolyte composition, secondary cell, and method for manufacturing electrolyte sheet

ABSTRACT

Provided is an electrolyte composition, comprising one or two or more polymers, oxide particles, at least one electrolyte salt selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt, and a solvent, wherein structural units constituting the one or two or more polymers comprises a first structural unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride, and a second structural unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate, a content of the one or two or more polymers is more than 90% by mass based on a total amount of polymers in the electrolyte composition, and a mass ratio of a content of the first structural unit to a content of the second structural unit in the one or two or more polymers is 50/50 or more.

TECHNICAL FIELD

The present invention relates to an electrolyte composition, a secondary battery, and a method for manufacturing an electrolyte sheet.

BACKGROUND ART

In recent years, with the widespread use of portable electronic devices, electric vehicles, and the like, high performance secondary batterys are required. Among such batterys, lithium secondary batterys have been attracting attention as a power source for electric vehicle batteries, power storage batteries, and the like because of their high energy density. Specifically, lithium secondary batterys as batteries for electric vehicles are employed in electric vehicles such as zero-emission electric vehicles that are not equipped with an engine, hybrid electric vehicles that are equipped with an engine and a secondary battery, and plug-in hybrid electric vehicles that are charged directly from the power system. In addition, lithium secondary batterys as power storage batteries are employed in, for example, stationary power storage systems that supply power stored in advance in an emergency when the power system is shut off.

In order to use in such a wide range of applications, there is a need for lithium secondary batterys having even higher energy density, and development of such batterys is being carried out. In particular, since lithium secondary batterys for electric vehicles require high safety in addition to high input/output characteristics and high energy density, more advanced technology for ensuring safety is required.

Conventionally, as a method for improving the safety of a lithium secondary battery, there are known a method for making an electrolyte solution flame-retardant by adding a flame retardant, a method for changing the electrolyte solution to a polymer electrolyte or a gel electrolyte, and the like. In particular, a gel electrolyte has the same ionic conductivity as the electrolyte solution used in conventional lithium secondary batterys; therefore, by changing the electrolyte solution to a gel electrolyte, combustion of the electrolyte solution can be suppressed by reducing the amount of the electrolyte solution that is released without degrading the battery performance.

Patent Literature 1 discloses a gel electrolyte layer containing a lithium salt-containing plasticizer, a matrix polymer in which a plasticizer is dispersed, and a fibrous insoluble material. The fibrous insoluble material contained in the gel electrolyte in an amount of 0.1% by weight or more and 50% by weight or less has a ratio of fiber length to fiber diameter of 10 or more and 3000 or less, a fiber length of 10 μm or more and 1 cm or less, and a fiber diameter of 0.05 μm or more and 50 μm or less; by setting in this manner, the cycle characteristics and high-temperature storage characteristics of the battery are improved.

Patent Literature 2 discloses a gel electrolyte and a gel electrolyte battery. The gel electrolyte layer is formed by swelling a matrix polymer with an electrolyte solution, and contains a large amount of a low viscosity solvent having a low boiling point. By using a gel electrolyte containing a large amount of low-viscosity solvent having a low boiling point, a gel electrolyte battery having excellent temperature characteristics, current characteristics, capacity, and charge/discharge characteristics at low temperatures is provided.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication No. 2000-164254

Patent Literature 2: Japanese Unexamined Patent Publication No. 2007-141467

SUMMARY OF INVENTION Technical Problem

However, there is still room for improvement in conventional gel electrolytes such as those described above. For example, in the invention described in Patent Literature 1, it is difficult to suitably form an electrolyte sheet (particularly, to make the electrolyte layer a thin layer) and to increase conductivity. Further, in the invention described in Patent Literature 2, the conductivity of the electrolyte is insufficient.

Therefore, it is an object of the present invention to provide an electrolyte composition having high conductivity and that is capable of obtaining an electrolyte sheet having excellent smoothness.

Solution to Problem

A first aspect of the present invention is an electrolyte composition, comprising: one or two or more polymers; oxide particles; at least one electrolyte salt selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt; and a solvent, wherein structural units constituting the one or two or more polymers comprise a first structural unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride, and a second structural unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate, a content of the one or two or more polymers is more than 90% by mass based on a total amount of polymers in the electrolyte composition, and a mass ratio of a content of the first structural unit to a content of the second structural unit in the one or two or more polymers is 50/50 or more.

This electrolyte composition has excellent conductivity, and is capable of obtaining an electrolyte sheet having excellent smoothness. Moreover, this electrolyte composition is suitable for the preparation of a slurry when forming into a sheet, and when the electrolyte composition is formed into a sheet, the obtained electrolyte sheet has excellent tensile strength. According to this electrolyte composition, it is possible to manufacture a lithium secondary battery having excellent discharge rate characteristics and high so-called initial characteristics in which the initial discharge capacity (initial capacity) is as close as possible to the design capacity.

The one or two or more polymers may comprise a copolymer comprising both the first structural unit and the second structural unit.

The one or two or more polymers may comprise at least two polymers of a first polymer comprising the first structural unit and a second polymer comprising the second structural unit.

The content of the one or two or more polymers is preferably 3 to 50% by mass based on the total amount of the electrolyte composition.

The oxide particles are preferably particles of at least one selected from the group consisting of SiO₂, Al₂O₃, AOOOH, MgO, CaO, ZrO₂, TiO₂, Li₇La₃Zr₂O₁₂, and BaTiO₃.

The content of the oxide particles is preferably 5 to 40% by mass based on the total amount of the electrolyte composition.

The solvent may be a glyme represented by the following formula (1),

R¹O—(CH₂CH₂O)_(k)—R²   (1)

wherein R¹ and R² each independently represent an alkyl group having 4 or less carbon atoms or a fluoroalkyl group having 4 or less carbon atoms, and k represents an integer of 1 to 6.

The solvent may be an ionic liquid.

The ionic liquid may comprise, as a cation component, at least one selected from the group consisting of a chain-like quaternary onium cation, a piperidinium cation, a pyrrolidinium cation, a pyridinium cation, and an imidazolium cation, and as an anion component, at least one of anion components represented by the following formula (2),

N(SO₂C_(m)F_(2m+1))(SO₂C_(n)F_(2n+1))⁻  (2)

wherein m and n each independently represent an integer of 0 to 5.

The total content of the electrolyte salt and the solvent is preferably 25 to 70% by mass based on the total amount of the electrolyte composition.

The electrolyte composition may be formed into a sheet.

A second aspect of the present invention is a secondary battery, comprising: a positive electrode; a negative electrode; and an electrolyte layer consisting of the electrolyte composition disposed between the positive electrode and the negative electrode.

A third aspect of the present invention is a method for manufacturing an electrolyte sheet, comprising: disposing on a substrate a slurry comprising one or two or more polymers, oxide particles, at least one electrolyte salt selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt, a solvent, and a dispersion medium; and volatilizing the dispersion medium to form an electrolyte layer on the substrate, wherein structural units constituting the one or two or more polymers comprises a first structural unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride, and a second structural unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate, a content of the one or two or more polymers is more than 90% by mass based on a total amount of polymers included in a solid content of the slurry, and a mass ratio of a content of the first structural unit to a content of the second structural unit in the one or two or more polymers is 50/50 or more.

Advantageous Effects of Invention

According to the present invention, there can be provided an electrolyte composition having high conductivity and that is capable of obtaining an electrolyte sheet having excellent smoothness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating a secondary battery according to a first embodiment.

FIG. 2 is an exploded perspective view for illustrating an embodiment of an electrode group in the secondary battery illustrated in FIG. 1.

FIG. 3 is a schematic cross-sectional view illustrating an embodiment of the electrode group in the secondary battery illustrated in FIG. 1.

FIG. 4(a) is a schematic cross-sectional view illustrating an electrolyte sheet according to one embodiment, and FIG. 4(b) is a schematic cross-sectional view illustrating an electrolyte sheet according to another embodiment.

FIG. 5 is a schematic cross-sectional view illustrating an embodiment of an electrode group in a secondary battery according to a second embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with appropriate reference to the drawings. However, the present invention is not limited to the following embodiments. In the following embodiments, the components (including steps and the like) are not essential unless otherwise specified. The size of the components in each figure is conceptual, and the relative relationship of the size between the components is not limited to that shown in each figure.

The numerical values and ranges thereof in present specification do not limit the present invention. In present specification, a numerical range indicated by using the word “to” indicates a range including the numerical values described before and after the word “to” as the minimum value and the maximum value, respectively. In the numerical ranges described in stages in present specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value described in another stage of the description. Moreover, in the numerical ranges described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the values shown in the Examples.

First Embodiment

FIG. 1 is a perspective view illustrating a secondary battery according to a first embodiment. As illustrated in FIG. 1, a secondary battery 1 includes an electrode group 2 constructed of a positive electrode, a negative electrode, and an electrolyte layer, and a bag-shaped battery outer casing 3 housing the electrode group 2. A positive electrode current collecting tab 4 and a negative electrode current collecting tab 5 are provided on the positive electrode and the negative electrode, respectively. The positive electrode current collecting tab 4 and the negative electrode current collecting tab 5 protrude from the inside of the battery outer package 3 to the outside such that the positive electrode and the negative electrode can each be electrically connected to the outside of the secondary battery 1.

The battery outer casing 3 may be formed of a laminate film, for example. The laminate film may be a laminate film in which a resin film such as a polyethylene terephthalate (PET) film, a metal foil such as aluminum, copper, and stainless steel, and a sealant layer such as polypropylene are laminated in that order.

FIG. 2 is an exploded perspective view illustrating an embodiment of the electrode group 2 in the secondary battery 1 illustrated in FIG. 1. FIG. 3 is a schematic cross-sectional view illustrating an embodiment of the electrode group 2 in the secondary battery 1 illustrated in FIG. 1. As illustrated in FIGS. 2 and 3, an electrode group 2A according to the present embodiment includes a positive electrode 6, an electrolyte layer 7, and a negative electrode 8 in this order. The positive electrode 6 includes a positive electrode current collector 9 and a positive electrode mixture layer 10 provided on the positive electrode current collector 9. The positive electrode current collecting tab 4 is provided on the positive electrode current collector 9. The negative electrode 8 includes a negative electrode current collector 11 and a negative electrode mixture layer 12 provided on the negative electrode current collector 11. The negative electrode current collecting tab 5 is provided on the negative electrode current collector 11.

The positive electrode current collector 9 may be formed of aluminum, stainless steel, titanium, or the like. Specifically, the positive electrode current collector 9 may be, for example, an aluminum perforated foil having holes with a hole diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, or the like. In addition to the above, the positive electrode current collector 9 may be formed of any material as long as it does not cause changes such as dissolution and oxidation during use of the battery; the shape, production method, and the like of the positive electrode current collector 9 are not limited.

The thickness of the positive electrode current collector 9 may be 10 to 100 μm; from the viewpoint of reducing the overall volume of the positive electrode, the thickness is preferably 10 to 50 μm; and from the viewpoint of winding the positive electrode with a small curvature when forming the battery, the thickness is more preferably 10 to 20 μm.

In one embodiment, the positive electrode mixture layer 10 contains a positive electrode active material, a conductive agent, and a binder.

The positive electrode active material may be LiCoO₂, LiNiO₂, LiMn₂O₄, LiMnO₃, LiMn₂O₃, LiMnO₂, Li₄Mn₅O₁₂, LiMn_(2-a)M¹ _(a)O₂ wherein M¹ is one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and a=0.01 to 0.2, Li₂Mn₃M²O₈ wherein M² is one selected from the group consisting of Fe, Co, Ni, Cu, and Zn, Li_(1-b)M³ _(b)Mn₂O₄ wherein M³ is one selected from the group consisting of Mg, B, Al, Fe, Co, Ni, Cr, Zn, and Ca, and b=0.01 to 0.1, LiFeO₂, Fe₂(SO₄)₃, LiCo_(1-c)M⁴ _(c)O₂ wherein M⁴ is one selected from the group consisting of Ni, Fe, and Mn, and c=0.01 to 0.2, LiNi_(1-d)M⁵ _(d)O₂ wherein M⁵ is one selected from the group consisting of Mn, Fe, Co, Al, Ga, Ca, and Mg, and d=0.01 to 0.2, Fe(MoO₄)₃, FeF₃, LiFePO₄, LiMnPO₄, and the like.

The positive electrode active material may be primary particles that have not been granulated or may be secondary particles that have been granulated.

The particle size of the positive electrode active material is adjusted to be equal to or less than the thickness of the positive electrode mixture layer 10. If there are coarse particles having a particle size equal to or more than the thickness of the positive electrode mixture layer 10 in the positive electrode active material, the coarse particles are removed in advance by sieving classification, wind classification, or the like, and a positive electrode active material having a particle size equal to or less than the thickness of the positive electrode mixture layer 10 is selected.

The average particle size of the positive electrode active material is, from the viewpoint of suppressing deterioration in the filling property of the positive electrode active material due to the decrease in the particle size and increasing an electrolyte salt retention ability, preferably 0.1 μm or more, more preferably 1 μm or more, further preferably 2 μm or more, and preferably 30 μm or less, 25 μm or less, 20 μm or less, 10 μm or less, or 8 μm or less. The average particle size of the positive electrode active material is the particle size (D50) when the ratio (volume fraction) with respect to the volume of the whole positive electrode active material is 50%. The average particle size (D50) of the positive electrode active material is calculated by measuring a suspension of the positive electrode active material suspended in water by a laser scattering method using a laser scattering particle size measuring device (e.g., Microtrac).

The content of the positive electrode active material may be, based on the total amount of the positive electrode mixture layer, 70% by mass or more, 80% by mass or more, or 85% by mass or more. The content of the positive electrode active material may be, based on the total amount of the positive electrode mixture layer, 99% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less.

The conductive agent may be a carbon material such as carbon black, acetylene black, graphite, carbon fiber, or carbon nanotube. These conductive agents are used alone or in combination of two or more.

The content of the conductive agent may be, based on the total amount of the positive electrode mixture layer, 0.1% by mass or more, 1% by mass or more, or 3% by mass or more. From the viewpoint of suppressing an increase in the volume of the positive electrode 6 and the accompanying decrease in the energy density of the secondary battery 1, the content of the conductive agent is, based on the total amount of the positive electrode mixture layer, preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 8% by mass or less.

The binder is not limited as long as it does not decompose on the surface of the positive electrode 6, but is, for example, a polymer. The binder may be a cellulose such as carboxymethyl cellulose, cellulose acetate, and ethyl cellulose, a polymer containing as a monomer unit at least one selected from the group consisting of tetrafluoroethylene, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate, or a rubber such as styrene-butadiene rubber, ethylene-propylene rubber, isoprene rubber, acrylic rubber, or fluororubber. The binder may be polyvinylidene fluoride, polyacrylic acid, polyimide, polyamide, or the like. The binder is preferably a copolymer containing tetrafluoroethylene and vinylidene fluoride as structural units, or a copolymer containing vinylidene fluoride and hexafluoropropylene as structural units.

The binder content may be, based on the total amount of the positive electrode mixture layer, 0.5% by mass or more, 1% by mass or more, or 3% by mass or more. The binder content may be, based on the total amount of the positive electrode mixture layer, 20% by mass or less, 15% by mass or less, 10% by mass or less, or 7% by mass or less.

The positive electrode mixture layer 10 may further contain an ionic liquid to be described later. In that case, the content of the ionic liquid is, based on the total amount of the positive electrode mixture layer, preferably 3% by mass or more, more preferably 5% by mass or more, and further preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, and further preferably 20% by mass or less.

From the viewpoint of further improving the conductivity, the thickness of the positive electrode mixture layer 10 is equal to or more than the average particle size of the positive electrode active material;

specifically, the thickness of the positive electrode mixture layer 10 is preferably 5 μm or more, more preferably 10 μm or more, further preferably 15 μm or more, and particularly preferably 20 μm or more. The thickness of the positive electrode mixture layer 10 is preferably 100 μm or less, more preferably 80 μm or less, further preferably 70 μm or less, and particularly preferably 50 μm or less. By setting the thickness of the positive electrode mixture layer 10 to 100 μm or less, imbalances in charging/discharging caused by variations in the charge level of the positive electrode active material near the surface of the positive electrode mixture layer 10 and near the surface of the positive electrode current collector 9 can be suppressed.

The density of the mixtures of the positive electrode mixture layer 10 is preferably 2 g/cm³ or more from the viewpoint of bringing the conductive agent and the positive electrode active material into close contact with each other and reducing the electronic resistance of the positive electrode mixture layer 10.

The negative electrode current collector 11 may be formed of aluminum, copper, stainless steel, titanium, nickel, an alloy thereof, or the like. Specifically, the negative electrode current collector 11 may be a rolled copper foil, for example, a copper perforated foil having holes with a hole diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, or the like. The negative electrode current collector 11 may be formed of any material other than the above, and its shape, production method, and the like are not limited.

The thickness of the negative electrode current collector 11 may be 10 to 100 μm; from the viewpoint of reducing the overall volume of the negative electrode, the thickness is preferably 10 to 50 μm; and from the viewpoint of winding the negative electrode with a small curvature when forming the battery, the thickness is more preferably 10 to 20 μm.

In one embodiment, the negative electrode mixture layer 12 contains a negative electrode active material and a binder.

As the negative electrode active material, a material commonly used in the field of energy devices can be used. Specific examples of the negative electrode active material include metal lithium, lithium alloys or other metal compounds, carbon materials, metal complexes, and organic polymer compounds. One of the above-mentioned examples may be used alone for the negative electrode active material, or two or more may be used in combination. The negative electrode active material is preferably a carbon material. Examples of carbon materials include graphite such as natural graphite (flake graphite etc.) and artificial graphite, amorphous carbon, carbon fiber, and carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. The negative electrode active material may also be, from the viewpoint of obtaining a larger theoretical capacity (e.g., 500 to 1500 Ah/kg), silicon, tin, or a compound (an oxide, nitride, alloy with another metal) containing these elements. When a material having a large capacity is used, the thickness of the negative electrode mixture layer 12 can be reduced, and the electrode area that can be housed in the secondary battery 1 can be increased. As a result, the resistance of the secondary battery 1 can be reduced, which enables higher output, and at the same time, the capacity of the secondary battery 1 can be increased as compared with the case where a graphite negative electrode is used.

The average particle size (D50) of the negative electrode active material is, from the viewpoint of obtaining a well-balanced negative electrode 8 in which an increase in irreversible capacity due to a decrease in particle size is suppressed and the electrolyte salt retention ability is increased, preferably 1 μm or more, more preferably 5 μm or more, and further preferably 10 μm or more, and preferably 50 μm or less, more preferably 40 μm or less, and further preferably 30 μm or less. The average particle size (D50) of the negative electrode active material is measured by the same method as that for the average particle size (D50) of the positive electrode active material.

The binder and its content may be the same as described above for the binder and its content in the positive electrode mixture layer 10.

The negative electrode mixture layer 12 may further contain a conductive agent from the viewpoint of further reducing the resistance of the negative electrode 8. The negative electrode mixture layer 12 may further contain an ionic liquid. The kind and content of the conductive agent and the ionic liquid may be the same as the kind and content of the conductive agent and the ionic liquid in the positive electrode mixture layer 10 described above.

From the viewpoint of further improving the conductivity, the thickness of the negative electrode mixture layer 12 is equal to or more than the average particle size of the negative electrode active material; specifically, the thickness of the negative electrode mixture layer 12 is preferably 10 μm or more, more preferably 15 μm or more, and further preferably 20 μm or more. The thickness of the negative electrode mixture layer 12 is preferably 100 μm or less, 80 μm or less, 70 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less. By setting the thickness of the negative electrode mixture layer 12 to 100 μm or less, imbalances in charging/discharging caused by variations in the charge level of the positive electrode active material near the surface of the negative electrode mixture layer 12 and near the surface of the negative electrode current collector 11 can be suppressed.

The density of the mixtures of the negative electrode mixture layer 12 is preferably 1 g/cm³ or more from the viewpoint of bringing the conductive agent and the negative electrode active material into close contact with each other and reducing the electronic resistance of the negative electrode mixture layer 12.

The electrolyte layer 7 consists of an electrolyte composition. The electrolyte composition contains one or two or more polymers, oxide particles, at least one electrolyte salt selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt, and a solvent. In the present specification, the electrolyte salt and the solvent may be collectively referred to as “electrolyte”, and the components of the electrolyte composition excluding the electrolyte may be collectively referred to as “electrolyte support material”.

Among the structural units (monomer units) constituting the one or two or more polymers, a first structural unit (monomer unit) selected from the group consisting of tetrafluoroethylene and vinylidene fluoride and a second structural unit (monomer unit) selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate are included.

The first structural unit and the second structural unit may be included in one polymer and constitute a copolymer. More specifically, in one embodiment, the electrolyte composition contains at least one copolymer including both the first structural unit and the second structural unit. The copolymer may be a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and maleic acid, a copolymer of vinylidene fluoride and methyl methacrylate, and the like. The copolymer may consist of only the first structural unit and the second structural unit, or may further contain a structural unit other than the first structural unit and the second structural unit. When the electrolyte composition contains a copolymer, it may contain only a copolymer containing the first structural unit and the second structural unit, or may further contain another polymer.

The first structural unit and the second structural unit may each be included in different polymers, and consist of at least two of a first polymer having the first structural unit and a second polymer having the second structural unit. Specifically, in one embodiment, the electrolyte composition contains at least two or more polymers of a first polymer including the first structural unit and a second polymer including the second structural unit. When the electrolyte composition contains the first polymer and the second polymer, the electrolyte composition may contain only the first polymer and the second polymer, or may also further contain another polymer.

The first polymer may be a polymer consisting of only the first structural unit, or may be a polymer further having another structural unit in addition to the first structural unit. Another structural unit may be an oxygen-containing hydrocarbon structure such as ethylene oxide (—CH₂CH₂O—) or a carboxylic acid ester (—CH₂COO—). The first polymer may be polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride, or a polymer in which the oxygen-containing hydrocarbon structure is incorporated inside those molecular structures.

The second polymer may be a polymer consisting of only the second structural unit, or may be a polymer further having another structural unit in addition to the second structural unit. Another structural unit may be an oxygen-containing hydrocarbon structure such as ethylene oxide (—CH₂CH₂O—) or a carboxylic acid ester (—CH₂COO—).

Examples of the combination of the first polymer and the second polymer include polyvinylidene fluoride and polyacrylic acid, polytetrafluoroethylene and polymethyl methacrylate, polyvinylidene fluoride and polymethyl methacrylate, and the like.

From the viewpoint of suppressing gelation of the electrolyte composition and suppressing the occurrence of unevenness when the electrolyte composition is formed into a sheet, it is preferable that the one or two or more polymers do not contain one or both of acrylonitrile and vinyl chloride as the structural unit (monomer unit). Specifically, the electrolyte composition may not contain a polymer including one or both of acrylonitrile and vinyl chloride as a structural unit (monomer unit). When the electrolyte composition contains a copolymer including the first structural unit and the second structural unit, the copolymer may not include one or both of acrylonitrile and vinyl chloride as the structural unit (monomer unit). When the electrolyte composition contains two or more polymers, the first polymer and the second polymer may not contain one or both of acrylonitrile and vinyl chloride as the structural unit (monomer unit), and another polymer other than the first polymer and the second polymer may not contain one or both of acrylonitrile and vinyl chloride as the structural unit (monomer unit).

From the viewpoint of further improving strength when the electrolyte composition is formed into a sheet, the content of the first structural unit is, based on the total content of the first structural unit and the second structural unit, preferably 50% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. From the viewpoint of further improving affinity with the solvent when the electrolyte composition contains a solvent, the content of the first structural unit is, based on the total content of the first structural unit and the second structural unit, preferably 99% by mass or less, 98% by mass or less, 97% by mass or less, or 96% by mass or less.

From the viewpoint of further improving strength when the electrolyte composition is formed into a sheet, the content of the first structural unit is, based on the total amount of the structural units constituting the polymer, preferably 5% by mass or more, 10% by mass or more, 20% by mass or more, 50% by mass or more, 70% by mass or more, or 90% by mass or more. From the viewpoint of further improving affinity with the solvent when the electrolyte composition contains a solvent, the content of the first structural unit is, based on the total amount of the structural units constituting the polymer, preferably 98% by mass or less, 95% by mass or less, or 90% by mass or less.

From the viewpoint of further improving affinity with the solvent when the electrolyte composition contains a solvent, the content of the second structural unit is, based on the total content of the first structural unit and the second structural unit, preferably 1% by mass or more, 3% by mass or more, or 4% by mass or more. From the viewpoint of further improving strength when the electrolyte composition is formed into a sheet, the content of the second structural unit is, based on the total content of the first structural unit and the second structural unit, preferably 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less.

From the viewpoint of further improving affinity with the solvent when the electrolyte composition contains a solvent, the content of the second structural unit is, based on the total amount of the structural units constituting the polymer, preferably 1% by mass or more, 3% by mass or more, or 5% by mass or more. From the viewpoint of further improving strength when the electrolyte composition is formed into a sheet, the content of the second structural unit is, based on the total amount of the structural units constituting the polymer, preferably 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

In the one or two or more polymers, the mass ratio of the content of the first structural unit to the content of the second structural unit (first structural unit content (mass)/second structural unit content (mass)) is 50/50 or more. The mass ratio of the content of the first structural unit to the content of the second structural unit is preferably 60/40 or more, 70/30 or more, 80/20 or more, 85/15 or more, 90/10 or more, 93/7 or more, or 95/5 or more. The mass ratio of the content of the first structural unit to the content of the second structural unit may be 99/1 or less, 97/3 or less, or 95/5 or less.

From the viewpoint of improving the conductivity of the electrolyte composition, the content of the one or two or more polymers is more than 90% by mass based on the total amount of polymer contained in the electrolyte composition. The content of the one or two or more polymers is, based on the total amount of polymer contained in the electrolyte composition, preferably 92% by mass or more, more preferably 94% by mass or more, and further preferably 96% by mass or more. The polymer contained in the electrolyte composition may consist of only the one or two or more polymers.

From the viewpoint of further improving strength when the electrolyte composition is formed into a sheet, the polymer content is, based on the total amount of the electrolyte composition, preferably 3% by mass or more, more preferably 5% by mass or more, further preferably 10% by mass or more, particularly preferably 20% by mass or more, and most preferably 25% by mass or more. From the viewpoint of further improving conductivity, the polymer content is, based on the total amount of the electrolyte composition, preferably 60% by mass or less, more preferably 50% by mass, further preferably 40% by mass or less, particularly preferably 30% by mass or less, and most preferably 28% by mass or less.

From the viewpoint of achieving both strength and conductivity when the electrolyte composition is formed into a sheet, the content of the polymer is, based on the total amount of the electrolyte composition, preferably 3 to 60% by mass, 3 to 50% by mass, 3 to 40% by mass, 3 to 30% by mass, 3 to 28% by mass, 5 to 60% by mass, 5 to 50% by mass, 5 to 40% by mass, 5 to 30% by mass, 5 to 28% by mass, 10 to 60% by mass, 10 to 50% by mass, 10 to 40% by mass, 10 to 30% by mass, 10 to 28% by mass, 20 to 60% by mass, 20 to 50% by mass, 20 to 40% by mass, 20 to 30% by mass, 20 to 28% by mass, 25 to 60% by mass, 25 to 50% by mass, 25 to 40% by mass, 25 to 30% by mass, or 25 to 28% by mass.

Since the polymer according to the present embodiment has excellent affinity with the solvent included in the electrolyte composition, the polymer retains the electrolyte in the solvent. As a result of this, the leakage of the solvent when a load is applied on the electrolyte composition is suppressed.

The oxide particles are, for example, particles of an inorganic oxide. The inorganic oxide may be an inorganic oxide containing, for example, Li, Mg, Al, Si, Ca, Ti, Zr, La, Na, K, Ba, Sr, V, Nb, B, Ge, and the like as constituent elements. The oxide particles are preferably particles of at least one selected from the group consisting of SiO₂, Al₂O₃, AOOOH, MgO, CaO, ZrO₂, TiO₂, Li₇La₃Zr₂O₂, and BaTiO₃. Since the oxide particles are polarity, the battery characteristics can be improved by promoting dissociation of the electrolyte in the electrolyte layer 7.

The oxide particles may be a rare earth metal oxide. Specifically, the oxide particles may be particles of scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, lutetium oxide, and the like.

The specific surface area of the oxide particles is 2 to 380 m²/g, and may be 5 to 100 m²/g, 10 to 80 m²/g, or 15 to 60 m²/g. When the specific surface area is 2 to 380 m²/g, a secondary battery using the electrolyte composition containing such oxide particles tends to have excellent discharge characteristics. From the same viewpoint, the specific surface area of the oxide particles may be 5 m²/g or more, 10 m²/g or more, or 15 m²/g or more, and 100 m²/g or less, 80 m²/g or less, or 60 m²/g or less. The specific surface area of the oxide particles means the specific surface area of the whole of the oxide particles, including primary particles and secondary particles, and is measured by a BET method.

From the viewpoint of further improving conductivity, the average primary particle size of the oxide particles is preferably 0.005 μm (5 nm) or more, more preferably 0.01 μm (10 nm) or more, and further preferably 0.015 μm (15 nm) or more. From the viewpoint of making the electrolyte layer 7 thin, the average primary particle size of the oxide particles is preferably 1 μm or less, more preferably 0.1 μm or less, and further preferably 0.05 μm or less. The average primary particle size of the oxide particles can be measured by a method similar to the average particle size (D50) of the positive electrode active material in a state where the particles are sufficiently crushed or dispersed. When it is difficult to crush or disperse the particles, the oxide particles may be observed with a scanning electron microscope to measure the average primary particle size by image analysis.

From the viewpoint of increasing the tensile strength when the electrolyte composition is formed into a sheet, the average primary particle size of the oxide particles is preferably 0.05 μm (50 nm) or more, more preferably 0.07 μm (70 nm) or more, further preferably 0.5 μm or more, and particularly preferably 1 μm or more, and preferably 5 μm or less, more preferably 4 μm or less, and further preferably 2 μm or less.

From the viewpoint of increasing the cross-sectional area in which the cation component of the electrolyte diffuses and further improving the conductivity, the average particle size (D50) of the oxide particles is preferably 0.001 μm or more, more preferably 0.005 μm or more, further preferably 0.05 μm or more, particularly preferably 0.1 μm or more, and most preferably 0.5 μm or more. The average particle size of the oxide particles is preferably 10 μm or less, more preferably 6 μm or less, further preferably 3μm or less, particularly preferably 2 μm or less, and most preferably 1μm or less. When the average particle size of the oxide particles is 6 μm or less, the electrolyte layer 7 can be suitably made thinner. More specifically, in this case, the oxide particles are less likely to aggregate, and as a result, the oxide particles can be prevented from protruding from the electrolyte layer 7 and damaging the surface of the positive electrode 6 and the surface of the negative electrode 8. In addition, since it is easier to ensure the thickness of the electrolyte layer 7, it is possible to suppress a reduction in the mechanical strength of the electrolyte layer 7. From the viewpoint of further improving conductivity by suppressing diffusion of lithium ions, the viewpoint of making the electrolyte composition thinner, and the viewpoint of suppressing protrusion of the oxide particles from the surface of the electrolyte composition, the average particle size of the oxide particles is preferably 0.001 to 10 μm; from the viewpoint of even more suitably making the electrolyte layer thinner, the average particle size of the oxide particles is more preferably 0.001 to 6μm; and from the viewpoint of preventing liquid leakage, the average particle size of the oxide particles is further preferably 0.001 to 0.1 μm or 0.005 to 1μm. The average particle size (D₅₀) of the oxide particles is measured by the same method as the average particle size (D₅₀) of the positive electrode active material.

The shape of the oxide particles may be, for example, a block shape or a substantially spherical shape. From the viewpoint of facilitating thinning of the electrolyte layer 7, the aspect ratio of the oxide particles is preferably 10 or less, more preferably 5 or less, and further preferably 2 or less. The aspect ratio is defined as the ratio of the length of the particles in the long axis direction (maximum length of the particles) and the length of the particles in the short axis direction (minimum length of the particles) calculated from a scanning electron micrograph of the oxide particles. The length of the particles is obtained by statistically calculating the above photograph by using commercially available image processing software (e.g., image analysis software by Asahi Kasei Engineering Corporation, AZO Kun (registered trademark)).

The oxide particles may be surface-treated with a silicon-containing compound. Specifically, the surface of the oxide particles may be bonded with the silicon atom of the silicon-containing compound via an oxygen atom. The silicon-containing compound is preferably at least one selected from the group consisting of an alkoxysilane, an epoxy group-containing silane, an amino group-containing silane, a silazane, and a siloxane.

The alkoxysilane may be methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, dimethoxydiphenylsilane, n-propyltrimethoxysilane, hexyltrimethoxysilane, tetraethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, n-propyltriethoxysilane, and the like.

The epoxy group-containing silane may be 2-(3,4-epoxycyclohexyl)pethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and the like.

The amino group-containing silane may be N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, and the like.

The silazane may be hexamethyldisilazane and the like. The siloxane may be dimethyl silicone oil and the like. These may be a silazane or siloxane having a reactive functional group (e.g., a carboxyl group) on one or both terminals thereof.

The surface-treated oxide particles may be particles produced by a known method, or a commercially available product may be used as is.

From the viewpoint of promoting dissociation of the electrolyte salt, the content of the oxide particles is, based on the total amount of the electrolyte composition, preferably 5% by mass or more, 10% by mass or more, 15% by mass or more, and 20% by mass or more. From the viewpoint of further improving conductivity, the content of the oxide particles is, based on the total amount of the electrolyte composition, preferably 50% by mass or less and more preferably 40% by mass or less. From the viewpoint of promoting dissociation of the electrolyte salt and further improving conductivity, the content of the oxide particles is, based on the total amount of the electrolyte composition, preferably 5 to 50% by mass, 5 to 40% by mass, 10 to 50% by mass, 10 to 40% by mass, 15 to 50% by mass, 15 to 40% by mass, 20 to 50% by mass, or 20 to 40% by mass.

The mass ratio of the polymer content to the oxide particle content (polymer content/oxide particle content) may be, for example, 2/3 or more, 1/1 or more, or 3/2 or more, and may be 4/1 or less, 3/1 or less, or 2/1 or less.

The electrolyte salt is at least one selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt. The electrolyte salt is a compound used to exchange cations between the positive electrode 6 and the negative electrode 8.

The above-mentioned electrolyte salts are preferable in that they have a low degree of dissociation at low temperatures, easily diffuse in solvents, and do not thermally decompose at high temperatures, and therefore the environmental temperature at which the secondary battery can be used is wide. The electrolyte salt may be an electrolyte salt used in a fluorine ion battery.

The anion of the electrolyte salt may be a halide ion (I⁻, Cl⁻, Br⁻, etc.), SCN⁻, BF₄ ⁻, BF₃(CF₃)⁻, BF₃(C₂F₅)⁻, PF₆ ⁻, ClO₄ ⁻, SbF₆ ⁻, N(SO₂F)₂ ⁻, N(SO₂CF₃)₂ ⁻, N(SO₂C₂F₅)₂ ⁻, B(C₆H₅)₄ ⁻, B(O₂C₂H₄)₂ ⁻, C(SO₂F)₃ ⁻, C(SO₂CF₃)₃ ⁻, CF₃COO, CF₃SO₂O⁻, C₆F₅SO₂O⁻, B(O₂C₂O₂)₂ ⁻¹, or the like. The anion is preferably PF₆ ⁻, BF₄ ⁻, N(SO₂F)₂ ⁻, N(SO₂CF3)₂ ⁻, B(O₂C₂O₂)₂ ⁻, or ClO₄ ⁻.

In the below description, the following abbreviations may be used.

-   [FSI]⁻: N(SO₂F)₂ ⁻, bis(fluorosulfonyl)imide anion -   [TFSI]⁻: N(SO₂CF₃)₂ ⁻, bis(trifluoromethanesulfonyl)imide anion -   [BOB]⁻: B(O₂C₂O₂)₂ ⁻, bisoxalate borate anion -   [f3C]⁻: C(SO₂F)₃ ⁻, tris(fluorosulfonyl)carbanion

The lithium salt may be at least one selected from the group consisting of LiPF₆, LiBF₄, Li[FSI], Li[TFSI], Li[f3C], Li[BOB], LiCl₄, LiBF₃(CF₃), LiBF₃(C₂F₅), LiBF₃(C₃F₇), LiBF₃(C₄F₉), LiC(SO₂CF₃)₃, LiCF₃SO₂O, LiCF₃COO, and LiRCOO wherein R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group.

The sodium salt may be at least one selected from the group consisting of NaPF₆, NaBF₄, Na[FSI], Na[TFSI], Na[f3C], Na[BOB], NaClO₄, NaBF₃(CF₃), NaBF₃(C₂F₅), NaBF₃(C₃F₇), NaBF₃(C₄F₉), NaC(SO₂CF₃)₃, NaCF₃SO₂O, NaCF₃COO, and NaRCOO wherein R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group.

The calcium salt may be at least one selected from the group consisting of Ca(PF₆)₂, Ca(BF₄)₂, Ca[FSI]₂, Ca[TFSI]₂, Ca[f3C]₂, Ca[BOB]₂, Ca(ClO)₂, Ca[BF₃(CF₃)]₂, Ca[BF₃(C₂F₅)]₂, Ca[BF₃(C₃F₇)]₂, Ca[BF₃(C₄F₉)]₂, Ca[C(SO₂CF₃)₃]₂, Ca(CF₃SO₂O)₂, Ca(CF₃COO)₂, and Ca(RCOO)₂ wherein R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group.

The magnesium salt may be at least one selected from the group consisting of Mg(PF₆)₂, Mg(BF₄)₂, Mg[FSI]₂, Mg[TFSI]₂, Mg[f3C]₂, Mg[BOB]₂, Mg(ClO₄)₂, Mg[BF₃(CF₃)]₂, Mg[BF₃(C₂F₅)]₂, Mg[BF₃(C₃F₇)]₂, Mg[BF₃(C₄F₉)]₂, Mg[C(SO₂CF₃)₃]₂, Mg(CF₃SO₃)₂, Mg(CF₃COO)₂, and Mg(RCOO)₂ wherein R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group.

Among these, from the viewpoint of dissociation and electrochemical stability, the electrolyte salt is preferably at least one selected from the group consisting of LiPF₆, LiBF₄, Li[FSI], Li[TFSI], Li[f3C], Li[BOB], LiClO₄, LiBF₃(CF₃), LiBF₃(C₂F₅), LiBF₃(C₃F₇), LiBF₃(C₄F₉), LiC(SO₂CF₃)₃, LiCF₃SO₂O, LiCF₃COO, and LiRCOO wherein R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group, and more preferably is at least one selected from the group consisting of Li[TFSI], Li[FSI], LiPF₆, LiBF₄, Li[BOB], and LiClO₄, and further preferably at least one selected from the group consisting of Li[TFSI], Li[FSI].

In order to suitably produce the electrolyte layer 7, the electrolyte salt content may be, based on the total amount of the electrolyte composition, 10% by mass or more and 60% by mass or less. The content of the electrolyte salt is preferably 20% by mass or more from the viewpoint of further increasing the conductivity of the electrolyte layer, and more preferably 30% by mass or more from the viewpoint of enabling charging/discharging of the lithium secondary battery at a high load factor.

The solvent preferably has a low vapor pressure and does not combust easily.

The solvent may be a glyme represented by the following formula (1).

R¹O—(CH₂CH₂O)_(k)—R²   (1)

In formula (1), R¹ and R² each independently represent an alkyl group having 4 or less carbon atoms or a fluoroalkyl group having 4 or less carbon atoms, k represents an integer of 1 to 6. R¹ and R² are each independently preferably a methyl group or an ethyl group.

Specifically, the glyme may be monoglyme (k=1), diglyme (k =2), triglyme (k=3), tetraglyme (k=4), pentaglyme (k=5), or hexaglyme (k=6).

When the electrolyte composition contains a glyme as a solvent, a part or all of the glyme may form a complex with the electrolyte salt.

The solvent may also be an ionic liquid. The ionic liquid contains the following anion component and cation component. Note that the ionic liquid in the present embodiment is a liquid substance at −20° C. or higher.

The anion component of the ionic liquid is not particularly limited, and may be a halogen anion such as Cl⁻, Br⁻, I⁻, an inorganic anion such as BF4⁻, N(SO₂F)₂ ⁻, an organic anion such as B(C₆H₅)₄ ⁻, CH₃SO₃ ⁻, CF₃SO₃ ⁻, N(C₄F₉SO₂)₂ ⁻, N(SO₂CF₃)₂ ⁻, N(SO₂C₄F₉)₂ ⁻, or the like. The anion component of the ionic liquid preferably contains at least one selected from the group consisting of B(C₆H₅)₄ ⁻, CH₃SO₃ ⁻, N(SO₂C₄F₉)₂, CF₃SO₂O^(−, N(SO) ₂F)₂ ⁻, N(SO₂CF₃)₂ ⁻, and N(SO₂C₂F₅)₂ ⁻; from the viewpoint of having a relatively low viscosity, further improving ionic conductivity, and further improving charge/discharge characteristics, it is more preferable to contain at least one selected from the group consisting of N(C₄F₉SO₂)₂ ⁻, CF₃SO₃ ⁻, N(SO₂F)₂ ⁻, N(SO₂CF₃)₂, and N(SO₂CF₂CF₃)₂, and further preferably N(SO₂F)₂ ⁻.

The anion component of the ionic liquid may contain at least one anion component represented by the following formula (2).

N(SO₂C_(m)F_(2m+1))(SO₂C_(n)F_(2n+1))⁻  (2)

m and n each independently represent an integer of 0 to 5. m and n may be the same or different from each other, and are preferably the same as each other.

Examples of the anion component represented by formula (2) include N(SO₂C₄F₉)₂ ⁻, N(SO₂F)₂ ⁻, N(SO₂CF₃)₂ ⁻, and N(SO₂C₂F₅)₂ ⁻.

The cation component of the ionic liquid is not particularly limited, but is preferably at least one selected from the group consisting of a chain-like quaternary onium cation, a piperidinium cation, a pyrrolidinium cation, a pyridinium cation, and an imidazolium cation.

The chain-like quaternary onium cation is, for example, a compound represented by the following formula (3).

wherein R³ to R⁶ each independently represent a chain-like alkyl group having 1 to 20 carbon atoms, or a chain-like alkoxyalkyl group represented by R—O—(CH₂)_(n)— wherein R represents a methyl group or an ethyl group, n represents an integer of 1 to 4, and X represents a nitrogen atom or a phosphorus atom; and the number of carbon atoms of the alkyl group represented by R³ to R⁶ is preferably 1 to 20, more preferably 1 to 10, and further preferably 1 to 5.

The piperidinium cation is, for example, a nitrogen-containing six-membered cyclic compound represented by the following formula (4).

wherein R⁷ and R⁸ each independently represent an alkyl group having 1 to 20 carbon atoms, or an alkoxyalkyl group represented by R—O—(CH₂)_(n)— wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4; and the number of carbon atoms of the alkyl group represented by R⁷ and R⁸ is preferably 1 to 20, more preferably 1 to 10, and further preferably 1 to 5.

The pyrrolidinium cation is, for example, a five-membered cyclic compound represented by the following formula (5).

wherein R⁹ and R¹⁰ each independently represent an alkyl group having 1 to 20 carbon atoms, or an alkoxyalkyl group represented by R—O—(CH₂)_(n)— wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4; and the number of carbon atoms of the alkyl group represented by R⁹ and R¹⁰ is preferably 1 to 20, more preferably 1 to 10, and further preferably 1 to 5.

The pyridinium cation is, for example, a compound represented by the following formula (6).

wherein R¹¹ to R¹⁵ each independently represent an alkyl group having 1 to 20 carbon atoms, an alkoxyalkyl group represented by R—O—(CH₂)_(n)— wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4, or a hydrogen atom; and the number of carbon atoms of the alkyl group represented by R¹¹ to R¹⁵ is preferably 1 to 20, more preferably 1 to 10, and further preferably 1 to 5.

The imidazolium cation is, for example, a compound represented by the following formula (7).

wherein R¹⁶ to R²⁰ each independently represent an alkyl group having 1 to 20 carbon atoms, an alkoxyalkyl group represented by R—O—(CH₂)_(n)— wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4, or a hydrogen atom; and the number of carbon atoms of the alkyl group represented by R¹⁶ to R²⁰ is preferably 1 to 20, more preferably 1 to 10, and further preferably 1 to 5.

More specifically, the ionic liquid may be

N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-bis(trifluoromet hanesulfonyl)imide (DEME-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-bis(fluorosulfon yl)imide (DEME-FSI), 1-ethyl-3-methylimidazolidium-bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolidium-bis(fluorosulfonyl)imide (EMI-FSI), N-methyl-N-propylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (Py13-TFSI), N-methyl-N-propylpyrrolidinium-bis(fluorosulfonyl)imide (Py13-FSI), N-ethyl-N-methylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (Py12-TFSI), N-ethyl-N-methylpyrrolidinium-bis(fluorosulfonyl)imide (Py12-FSI), or 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA).

The electrolyte composition may contain, as a solvent, for the purpose of further improving the conductivity, a nonaqueous solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, y-butyrolactone, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphoric acid triester, trimethoxymethane, dioxolane, diethyl ether, sulfolane, 3-methyl-2-oxazolidinone, tetrahydrofuran, 1,2-diethoxyethane, chloroethylene carbonate, and chloropropylene carbonate. From the viewpoint of improving safety, the electrolyte composition preferably contains only at least one solvent selected from the above-mentioned glymes and ionic liquids as a solvent.

From the viewpoint of suitably producing the electrolyte layer, the content of the solvent may be, based on the total amount of the electrolyte composition, 10 to 80% by mass, or 10 to 60% by mass or less. From the viewpoint of suppressing a reduction in the strength of the electrolyte layer, the content of the solvent is preferably 80% by mass or less based on the total amount of the electrolyte layer. From the viewpoint of increasing the conductivity of the electrolyte membrane by increasing the electrolyte salt content to thereby enable charging/discharging of the lithium secondary battery at a high load factor, the content of the solvent may be, based on the total amount of the electrolyte composition, 40% by mass or less, or 30% by mass or less.

From the viewpoint of further improving the conductivity and suppressing a reduction in the capacity of the secondary battery, the total content of the electrolyte salt and the solvent (e.g., glyme such as tetraglyme or ionic liquid) is, based on the total amount of the electrolyte composition, preferably 10% by mass or more, more preferably 25% by mass or more, and further preferably 40% by mass or more. From the viewpoint of suppressing a reduction in the strength of the electrolyte composition, the total content of the electrolyte salt and the solvent is preferably 80% by mass or less and more preferably 70% by mass or less. From the viewpoint of further improving the conductivity and suppressing a reduction in the capacity of the secondary battery and the strength of the electrolyte composition, the total content of the electrolyte salt and the solvent is, based on the total amount of the electrolyte composition, preferably 10 to 80% by mass, 10 to 70% by mass, 25 to 80% by mass, 25 to 70% by mass, 40 to 80% by mass, or 40 to 70% by mass.

In the electrolyte composition, from the viewpoint of further improving charge/discharge characteristics, the concentration of the electrolyte salt per unit volume of the solvent is preferably 0.5 mol/L or more, more preferably 0.7 mol/L or more, further preferably 0.8 mol/L or more, and preferably 2.0 mol/L or less, more preferably 1.8 mol/L or less, and further preferably 1.5 mol/L or less.

The mass ratio of the electrolyte content to the electrolyte support material content (electrolyte content/electrolyte support material content) may be, from the viewpoint of suppressing leakage of the electrolyte composition, ¼ or more or ⅔ or more, and 3/1 or less, 3/2 or less, or 1/1 or less.

The thickness of the electrolyte layer 7 is preferably 5 μm or more and more preferably 10 μm or more from the viewpoint of increasing conductivity and improving strength. From the viewpoint of suppressing the resistance of the electrolyte layer 7, the thickness of the electrolyte layer 7 is preferably 200 μm or less, more preferably 150 μm or less, further preferably 100 μm or less, and particularly preferably 50 μm or less.

Next, a method of manufacturing the secondary battery 1 described above is described. The manufacturing method of the secondary battery 1 according to the present embodiment includes a first step of forming the positive electrode mixture layer 10 on the positive electrode current collector 9 to obtain the positive electrode 6, a second step of forming the negative electrode mixture layer 12 on the negative electrode current collector 11 to obtain the negative electrode 8, and a third step of disposing the electrolyte layer 7 between the positive electrode 6 and the negative electrode 8.

In the first step, the positive electrode 6 is obtained by, for example, dispersing the material to be used for the positive electrode mixture layer in a dispersion medium using a kneading machine, a dispersing machine, or the like to obtain a slurry-like positive electrode mixture, then coating the positive electrode current collector 9 with the positive electrode mixture by a doctor blade method, a dipping method, a spray method or the like, and then volatilizing the dispersion medium.

After volatilizing the dispersion medium, a compression molding step using a roll press may be optionally carried out. The positive electrode mixture layer 10 may be formed as a positive electrode mixture layer having a multilayer structure by performing the above-mentioned steps from coating with the positive electrode mixture to volatilization of the dispersion medium a plurality of times.

The dispersion medium used in the first step may be water, 1-methyl-2-pyrrolidone (hereinafter also referred to as NMP), or the like. The dispersion medium is a compound other than the above-mentioned solvent.

When the positive electrode mixture layer 10 includes the positive electrode active material, the conductive agent, the binder, the electrolyte salt, and the solvent, the mixing ratio of the positive electrode active material, the conductive agent, the binder, and the solvent in which the electrolyte salt is dissolved may be, for example, positive electrode active material:conductive agent:binder:solvent in which electrolyte salt is dissolved=69 to 82:0.1 to 10:1 to 12:10 to 17 (mass ratio). However, the ratio is not necessarily limited to this range.

In the second step, the method of forming the negative electrode mixture layer 12 on the negative electrode current collector 11 may be the same method as the first step described above.

When the negative electrode mixture layer 12 includes the negative electrode active material, the conductive agent, the binder, the electrolyte salt, and the solvent, the mixing ratio of the negative electrode active material, the conductive agent, the binder, and the solvent in which the electrolyte salt is dissolved may be, for example, negative electrode active material:conductive agent:binder:ionic liquid in which electrolyte salt is dissolved=69 to 82:0.1 to 10:1 to 12:10 to 17 (mass ratio). However, the ratio is not necessarily limited to this range.

In the third step, in one embodiment, the electrolyte layer 7 is formed, for example, by manufacturing an electrolyte sheet provided with an electrolyte composition on a substrate. FIG. 4(a) is a schematic cross-sectional view illustrating an electrolyte sheet according to an embodiment. As illustrated in FIG. 4(a), an electrolyte sheet 13A includes a substrate 14 and an electrolyte layer 7 provided on the substrate 14.

The electrolyte sheet 13A is manufactured, for example, by dispersing a material (solid content) used for the electrolyte layer 7 in a dispersion medium to obtain a slurry, then coating the substrate 14 with the slurry and volatilizing the dispersion medium. The dispersion medium is preferably water, NMP, toluene, or the like.

The substrate 14 has heat resistance capable of withstanding the heating when the dispersion medium is volatilized, and is not limited as long as it does not react with the electrolyte composition and is not caused to swell by the electrolyte composition, and is formed, for example of a resin. Specifically, the substrate 14 may be a film consisting of a resin (general-purpose engineering plastic) such as polyethylene terephthalate, polytetrafluoroethylene, polyimide, polyethersulfone, polyetherketone, and the like.

The substrate 14 only needs to have a heat-resistant temperature that is capable of withstanding the processing temperature for volatilizing the dispersion medium in the process of producing the electrolyte layer. When the substrate 14 is formed of a resin, the heat-resistant temperature is a lower temperature than the softening point (temperature at which plastic deformation starts) or the melting point of the substrate 14. The heat-resistant temperature of the substrate 14 may be, from the viewpoint of compatibility with the solvent used for the electrolyte layer 7, preferably 50° C. or higher, more preferably 100° C. or higher, and further preferably 150° C. or higher, and for example, 400° C. or lower. When a substrate having the above heat-resistant temperature is used, a dispersion medium (NMP, toluene, etc.) such as described above can be suitably used.

The thickness of the substrate 14 is preferably as thin as possible while maintaining a strength capable of withstanding the tensile force of the coating apparatus. From the viewpoint of securing strength when the substrate 14 is coated with the electrolyte composition while reducing the volume of the entire electrolyte sheet 13A, the thickness of the substrate 14 is preferably 5 μm or more, more preferably 10 μm or more, and further preferably 25 μm or more, and preferably 100 μm or less, more preferably 50 μm or less, and further preferably 40 μm or less.

The electrolyte sheet can also be manufactured continuously while being rolled into a roll shape. In that case, the electrolyte layer 7 may be damaged as a result of the surface of the electrolyte layer 7 coming into contact with the back surface of the substrate 14 and a part of the electrolyte layer 7 sticking to the substrate 14. In order to prevent such a situation, as another embodiment, the electrolyte sheet may be provided with a protective material on the side opposite to the substrate 14 of the electrolyte layer 7. FIG. 4(b) is a schematic cross-sectional view illustrating an electrolyte sheet according to another embodiment. As illustrated in FIG. 4(b), an electrolyte sheet 13B further includes a protective material 15 on the opposite side of the electrolyte layer 7 from the substrate 14.

The protective material 15 may be any material that can be easily peeled off from the electrolyte layer 7; the protective material 15 is preferably a nonpolar resin film such as polyethylene, polypropylene, or polytetrafluoroethylene. When a nonpolar resin film is used, the electrolyte layer 7 and the protective material 15 do not stick to each other, and the protective material 15 can be easily peeled off.

From the viewpoint of ensuring strength while reducing the volume of the entire electrolyte sheet 13B, the thickness of the protective material 15 is preferably 5 μm or more, more preferably 10 μm, and preferably 100 μm or less, more preferably 50 μm or less, and further preferably 30 μm or less.

The heat-resistant temperature of the protective material 15 is, from the viewpoint of suppressing deterioration in a low temperature environment and suppressing softening in a high temperature environment, preferably −30° C. or higher and more preferably 0° C. or higher, and preferably 100° C. or lower and more preferably 50° C. or lower. When the protective material 15 is provided, it is not necessary to increase the heat-resistant temperature because the above-mentioned step of volatilizing the dispersion medium is not essential.

As a method of providing the electrolyte layer 7 between the positive electrode 6 and the negative electrode 8 using the electrolyte sheet 13A, for example, the substrate 14 is peeled from the electrolyte sheet 13A, and the positive electrode 6, the electrolyte layer 7, and the negative electrode 8 are stacked by, for example, lamination to obtain the secondary battery 1. At this time, the electrolyte layer 7 is positioned on the positive electrode mixture layer 10 side of the positive electrode 6 and on the negative electrode mixture layer 12 side of the negative electrode 8, that is, the positive electrode current collector 9, the positive electrode mixture layer 10, the electrolyte layer 7, the negative electrode mixture layer 12, and the negative electrode current collector 11 are stacked in that order.

Second Embodiment

Next, a secondary battery according to a second embodiment will be described. FIG. 5 is a schematic cross-sectional view illustrating an embodiment of an electrode group in the secondary battery according to the second embodiment. As illustrated in FIG. 5, the secondary battery of the second embodiment is different from the secondary battery of the first embodiment in that an electrode group 2B includes a bipolar electrode 16. Specifically, the electrode group 2B includes the positive electrode 6, a first electrolyte layer 7, a bipolar electrode 16, a second electrolyte layer 7, and the negative electrode 8 in this order.

The bipolar electrode 16 includes a bipolar electrode current collector 17, the positive electrode mixture layer 10 provided on the negative electrode 8 side (positive electrode surface) of a bipolar electrode current collector 17, and the negative electrode mixture layer 12 provided on a surface on the positive electrode 6 side (negative electrode surface) of a bipolar electrode current collector 17.

In the bipolar current collector 17, it is preferable that the positive electrode surface be formed of a material with excellent oxidation resistance, and may be formed of aluminum, stainless steel, titanium, or the like. The negative electrode surface in the bipolar current collector 17 in which graphite or an alloy is used as the negative electrode active material may be formed of a material that does not form an alloy with lithium, specifically, stainless steel, nickel, iron, titanium, or the like. When different metals are used for the positive electrode surface and the negative electrode surface, the bipolar current collector 17 may be a cladding material in which different metal foils are laminated. However, when a negative electrode 8 operating at a potential at which an alloy with lithium does not form, such as lithium titanate, is used, the above-mentioned limitation does not exist, and the negative electrode surface may be the same material as the positive electrode current collector 9. In that case, the bipolar current collector 17 may be a single metal foil. The bipolar current collector 17 as a single metal foil may be an aluminum perforated foil having holes with a hole diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, or the like. In addition to the above, the bipolar current collector 17 may be formed of any material as long as it does not cause changes such as dissolution and oxidation during use of the battery; and the shape, production method, and the like of the bipolar current collector 17 are not limited.

The thickness of the bipolar current collector 17 may be 10 μm or more and 100 μm or less; from the viewpoint of reducing the volume of the overall positive electrode, the thickness is preferably 10 μm or more and 50 μm or less; and from the viewpoint of winding the bipolar electrode with a small curvature when forming the battery, the thickness is more preferably 10 μm or more and 20 μm or less.

Next, a method of manufacturing the secondary battery according to the second embodiment will be described. The manufacturing method of the secondary battery according to the present embodiment includes a first step of forming the positive electrode mixture layer 10 on the positive electrode current collector 9 to obtain the positive electrode 6, a second step of forming the negative electrode mixture layer 12 on the negative electrode current collector 11 to obtain the negative electrode 8, a third step of forming the positive electrode mixture layer 10 on one side of the bipolar current collector 17 and the negative electrode mixture layer 12 on the other side to obtain the bipolar electrode 16, and a fourth step of disposing the electrolyte layer 7 between the positive electrode 6 and the bipolar electrode 16 and between the negative electrode 8 and the bipolar electrode 16.

The first step and the second step may be the same methods as the first step and the second step in the first embodiment.

In the third step, the method of forming the positive electrode mixture layer 10 on one surface of the bipolar current collector 17 may be the same method as the first step in the first embodiment. The method of forming the negative electrode mixture layer 12 on the other surface of the bipolar current collector 17 may be the same method as the second step in the first embodiment.

As the method of disposing the electrolyte layer 7 between the positive electrode 6 and the bipolar electrode 16 in the fourth step, in one embodiment, for example, the electrolyte layer 7 is formed by producing an electrolyte sheet having an electrolyte composition on a substrate. The manufacturing method of the electrolyte sheet may be the same method as the manufacturing method of the electrolyte sheets 13A and 13B in the first embodiment.

In the fourth step, the method of disposing the electrolyte layer 7 between the negative electrode 8 and the bipolar electrode 16 may be the same as the method of disposing the electrolyte layer 7 between the positive electrode 6 and the bipolar electrode 16 described above.

EXAMPLES

Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to these Examples.

Test Example 1 Example 1-1

A slurry containing an electrolyte composition was prepared by dispersing in an NMP, which is a dispersion medium, 26% by mass of a copolymer of vinylidene fluoride and hexafluoropyrene (vinylidene fluoride/hexafluoropyrene (mass ratio)=95/5, hereinafter also referred to as PVDF-HFP) as a polymer, 13% by mass of SiO₂ particles (average particle size 0.1 μm) as oxide particles, 34.5% by mass of lithium bis(fluorosulfonyl)imide (Li[FSI]) as an electrolyte salt, and 26.5% by mass of tetraglyme as a solvent. A polyethylene terephthalate substrate was coated with the obtained slurry, and heated to volatilize the dispersion medium to obtain an electrolyte sheet. The thickness of the electrolyte layer in the obtained electrolyte sheet was 25±2 μm.

Examples 1-2 to 1-4

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the content of each material in the electrolyte sheet of Example 1 was changed to the content shown in Table 1.

Example 1-5

An electrolyte sheet was manufactured in the same manner as in Example 1, except that the content of the oxide particles was increased and the total content of the electrolyte salt and the solvent ((A)+(B) in Table 1) was decreased from that in the electrolyte sheet of Example 1.

Examples 1-6 to 1-13

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the type of the oxide particles in the electrolyte sheet of Example 1 was changed to the type shown in Tables 1 to 2.

Examples 1-14 to 1-16

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the average particle size of the SiO₂ particles as oxide particles in the electrolyte sheet of Example 1 was changed to the average particle size shown in Table 2.

Examples 1-17 to 1-18

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the content of hexafluoropropylene in the polymer in the electrolyte sheet of Example 1 was changed to the content shown in Table 2.

Examples 1-19 to 1-22

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the solvent in the electrolyte sheet of Example 1 was changed to the solvent shown in Tables 2 to 3. In Table 3, the EMI-TFSI (EMI-BTI) of Example 1-21 represents 1-ethyl-3-methylimidazolidium-bis(trifluoromethanesulfonyl)imide [CAS No. 174899-82-2], and the EMI-DCA of Example 1-22 represents 1-ethyl-3-methylimidazolium dicyanamide [CAS No. 370865-89-7].

Examples 1-23 to 1-30

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the type of polymer in the electrolyte sheet of Example 1 was changed to type shown in Table 3. In Table 3, the abbreviations of the polymers represent the polymers described in the following, and polymer mixing ratio means mass ratio.

PVDF+PA: Mixture of polyvinylidene fluoride and polyacrylic acid

PVDF-MA: Copolymer of vinylidene fluoride and maleic acid PTFE+PMMA: Mixture of polytetrafluoroethylene and polymethyl methacrylate

PVDF+PMMA: Mixture of polyvinylidene fluoride and polymethyl methacrylate

PVDF-HFP+PMMA: Mixture of a copolymer of vinylidene fluoride and hexafluoropyrene with polymethyl methacrylate

PVDF-HFP+PAN: Mixture of a copolymer of vinylidene fluoride and hexafluoropyrene with polyacrylonitrile

PVDF-HFP+PVC: Mixture of a copolymer of vinylidene fluoride and hexafluoropyrene with polyvinyl chloride

Comparative Example 1-1

An electrolyte sheet was manufactured in the same manner as in Example 1, except that oxide particles were not used in the electrolyte sheet of Example 1-1.

Comparative Examples 1-2 to 1-8

Electrolyte sheets were manufactured in the same manner as in Example 1, except that the type and/or mixing ratio of the polymer in the electrolyte sheet of Example 1-1 was changed to those shown in Table 4. In Table 4, the abbreviations of the polymers represent the polymers described in the following, and polymer mixing ratio means mass ratio.

PVDF: Polyvinylidene fluoride

PVDF-HFP+PAN+PVC: Mixture of a copolymer of vinylidene fluoride and hexafluoropyrene with polyacrylonitrile and polyvinyl chloride

TABLE 1 Polymer Oxide Particles Electrolyte Solvent Polymer Type Polymer Particle Type Salt Content Content (mass ratio of Mixing Content (average particle Content (% by mass) Solvent (% by mass) structural units) Ratio (% by mass) size μm) (% by mass) (A) Type (B) (A) + (B) Example 1-1 PVDF-HFP — 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-2 PVDF-HFP — 30 SiO₂ (0.1) 15 31.1 tetraglyme 23.9 55 (95/5) Example 1-3 PVDF-HFP — 36 SiO₂ (0.1) 18 26 tetraglyme 20 46 (95/5) Example 1-4 PVDF-HFP — 13 SiO₂ (0.1) 26 34.5 tetraglyme 26.5 61 (95/5) Example 1-5 PVDF-HFP — 26 SiO₂ (0.1) 18 31.6 tetraglyme 24.4 56 (95/5) Example 1-6 PVDF-HFP — 26 Al₂O₃ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-7 PVDF-HFP — 26 AlOOH (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-8 PVDF-HFP — 26 MgO (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-9 PVDF-HFP — 26 CaO (O.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-10 PVDF-HFP — 26 ZrO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5)

TABLE 2 Polymer Oxide Particles Electrolyte Solvent Polymer Type Polymer Particle Type Salt Content Content (mass ratio of Mixing Content (average particle Content (% by mass) Solvent (% by mass) structural units) Ratio (% by mass) size μm) (% by mass) (A) Type (B) (A) + (B) Example 1-11 PVDF-HFP — 26 TiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-12 PVDF-HFP — 26 Li₇La₃Zr₂O₁₂ 13 34.5 tetraglyme 26.5 61 (95/5) (0.1) Example 1-13 PVDF-HFP — 26 BaTiO₃ 13 34.5 tetraglyme 26.5 61 (95/5) (0.1) Example 1-14 PVDF-HFP — 26 SiO₂ (1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-15 PVDF-HFP — 26 SiO₂ (3) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-16 PVDF-HFP — 26 SiO₂ (6) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-17 PVDF-HFP — 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (98/2) Example 1-18 PVDF-HFP — 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (97/3) Example 1-19 PVDF-HFP — 26 SiO₂ (0.1) 13 34.5 triglyme 26.5 61 (95/5) Example 1-20 PVDF-HFP — 26 SiO₂ (0.1) 13 34.5 diglyme 26.5 61 (95/5)

TABLE 3 Polymer Oxide Particles Electrolyte Solvent Polymer Type Polymer Particle Type Salt Content Content (mass ratio of Mixing Content (average particle Content (% by mass) Solvent (% by mass) structural units) Ratio (% by mass) size μm) (% by mass) (A) Type (B) (A) + (B) Example 1-21 PVDF-HFP — 26 SiO2 (0.1) 13 34.5 EMI-TFSI 26.5 61 (95/5) Example 1-22 PVDF-HFP — 26 SiO2 (0.1) 13 34.5 EMI-DCA 26.5 61 (95/5) Example 1-23 PVDF + PA 95/5 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-24 PVDF-MA — 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) Example 1-25 PTFE + PMMA  90/10 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-26 PVDF + PMMA 95/5 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-27 PVDF-HFP 98/2 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (98/2) + PMMA Example 1-28 PVDF-HFP  50/50 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (98/2) + PMMA Example 1-29 PVDF-HFP 91/9 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) + PAN Example 1-30 PVDF-HFP 91/9 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 (95/5) + PVC

TABLE 4 Polymer Oxide Particles Electrolyte Solvent Polymer Type Polymer Particle Type Salt Content Content (mass ratio of Mixing Content (average particle Content (% by mass) Solvent (% by mass) structural units) Ratio (% by mass) size μm) (% by mass) (A) Type (B) (A) + (B) Comparative PVDF-HFP — 39 — — 34.5 tetraglyme 26.5 61 Example 1-1 (95/5) Comparative PVDF — 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-2 Comparative PVDF-HFP 90/10 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-3 (95/5) + PAN Comparative PVDF-HFP 70/30 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-4 (95/5) + PVC Comparative PVDF-HFP 90/7/3 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-5 (95/5) + PAN + PVC Comparative PVDF-HFP 50/50 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-6 (95/5) + PAN Comparative PVDF-HFP 33/50/17 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-7 (95/5) + PAN + PVC Comparative PVDF-HFP 40/60 26 SiO₂ (0.1) 13 34.5 tetraglyme 26.5 61 Example 1-8 (95/5) + PMMA

[Evaluation of Electrolyte Sheet]

The following items were evaluated for the properties of the electrolyte sheets according to the Examples and Comparative Examples. The results are shown in Table 5.

(Conductivity)

A commercially available conductivity meter was introduced into the slurry in the electrolyte sheet preparation process, and the conductivity was measured at 25° C.

(Slurry Properties)

For the properties of the slurry in the preparation process of the electrolyte sheet, the presence or absence of gelation was confirmed visually. After the slurry was produced, the slurry was left in a dry room; after 5 hours had elapsed, in cases in which not even a part of the slurry had solidified (gelled), it was determined that the slurry had not gelled, and in cases in which at least a part had solidified (gelled), it was determined that the slurry had gelled. Slurry gelation can occur when the polymer, electrolyte salt, and solvent have low mutual solubility in the dispersion medium. When gelation occurs, it is difficult to make the electrolyte sheet thin, and therefore it is preferable that gelation does not occur. In Table 5, cases in which gelation occurred are shown by a “+”, and gelation did not occur are shown by a “−”.

(Surface Properties of Electrolyte Composition)

The surface properties of the electrolyte composition in the electrolyte sheet were visually confirmed. It is preferable that the surface of the electrolyte composition be smooth and free from unevenness and defects. In Table 5, cases having unevenness are shown by a “+”, and cases not having unevenness are shown by a “−”.

(Presence/Absence of Fracture)

The presence or absence of fracture when, in the electrolyte sheet, the end of the electrolyte layer was peeled from the substrate was visually evaluated. In Table 5, cases in which fracture occurred are shown by a “+”, and cases in which fracture did not occur are shown by a “−”.

(Presence/Absence of Liquid Leakage)

The presence or absence of liquid leakage due to the solvent when a load was applied on the electrolyte sheet was evaluated. A resin film (a film made of polyethylene terephthalate having a thickness of 25 μm) was pressed against the cut electrolyte sheet while applying a load of 2 kg/cm². At this time, the presence or absence of adhesion of the liquid electrolyte composition was visually evaluated. In Table 5, cases in which liquid leakage occurred are shown by a “+”, and cases in which liquid leakage did not occur are shown by a “−”.

TABLE 5 Presence/ Presence/ Presence/ Absence of Presence/ Conductivity Absence of Absence of Fracture Absence of (mS/cm) Slurry Gelation Unevenness During Peeling Liquid Leakage Example 1-1 0.8 − − − − Example 1-2 0.2 − − − − Example 1-3 0.1 − − − − Example 1-4 0.85 − − + + Example 1-5 0.4 − − − − Example 1-6 0.7 − − − − Example 1-7 0.7 − − − − Example 1-8 0.6 − − − − Example 1-9 0.6 − − − − Example 1-10 0.7 − − − − Example 1-11 0.7 − − − − Example 1-12 0.9 − − − − Example 1-13 0.6 − − − − Example 1-14 1 − − − − Example 1-15 0.7 − − − − Example 1-16 0.5 − − − − Example 1-17 0.4 − − − − Example 1-18 0.6 − − − − Example 1-19 0.7 − − − − Example 1-20 0.6 − − − − Example 1-21 1.2 − − − − Example 1-22 1.5 − − − − Example 1-23 0.5 − − − − Example 1-24 0.6 − − − − Example 1-25 0.6 − − − − Example 1-26 0.7 − − − − Example 1-27 1 − − − − Example 1-28 1.1 − − − − Example 1-29 0.08 − − − − Example 1-30 0.1 − − − − Comparative 0.03 − − + + Example 1-1 Comparative 0.03 + + + + Example 1-2 Comparative 0.02 + + + − Example 1-3 Comparative 0.01 + + + − Example 1-4 Comparative 0.03 + + + − Example 1-5 Comparative 0.008 + + + + Example 1-6 Comparative 0.007 + + + + Example 1-7 Comparative − − + + + Example 1-8

A comparison of the conductivity of Examples 1-1 to 1-4 in which the content of each material was changed showed a tendency for the conductivity to increase the larger the total content ((A)+(B)) of the electrolyte salt and the solvent is, that is, the larger the amount of the electrolyte salt added is.

Example 1-5, in which the content of oxide particles was increased and the content of (A)+(B) was decreased compared to Example 1-1, showed a slightly lower conductivity due to the decrease in electrolyte salt content than Example 1-1, but has a higher conductivity than Example 1-2, which contained almost the same amount of electrolyte salt. In other words, it was found that conductivity is further improved by increasing the content of the oxide particles.

Examples 1-6 to 1-13, in which the type of oxide particles was changed, had a conductivity of 0.6 mS/cm or more regardless of the oxide particles that were used.

Examples 1-14 to 1-16, in which the average particle size of the oxide particles was changed, exhibited, as compared with the Examples including Example 1-1, about the same conductivity as Example 1-14 and Example 1-15, in which the average particle size of the oxide particles was 0.1 μm to 3 μm, but it was found that the conductivity of Example 1-16, in which the average particle size of the oxide particles was 6μm, tended to decrease slightly. From this result, it can be seen that although it is possible to obtain an electrolyte sheet having excellent strength and conductivity by using oxide particles having an average particle size of up to about 6 μm, in order to further improve conductivity, the average particle size is desirably 1μm or less.

Examples 1-17 to 1-18, in which the content of hexafluoropropylene in the polymer was changed, showed that, as compared with the Examples including Example 1-1, conductivity tended to increase as the content of hexafluoropropylene increased.

Among Examples 1-19 to 1-22 in which the type of solvent was changed, in Examples 1-21 and 1-22, the conductivity exceeded 1 mS/cm.

Regarding Examples 1-23 to 1-30, in which the type of polymer was changed, Example 1-28 using three polymers consisting of a first structural unit and a second structural unit, had the highest conductivity.

The conductivity of Comparative Example 1-1, which did not contain oxide particles, was inferior to that of Examples 1-1 to 1-30.

The conductivity of Comparative Example 1-2, in which a polymer that did not contain the second structural unit was used, and

Comparative Examples 1-3 to 1-7, in which the content of the polymer containing the first structural unit and the second structural unit was 90% by mass or less based on the total amount of the polymer, were inferior to the conductivity of Examples 1-1 to 1-30. In addition, the slurries in Comparative Examples 1-2 to 1-7 gelled, and unevenness occurred on the surface of the electrolyte composition. Regarding liquid leakage, the larger the content of the polymer (PVDF-HFP) containing the first structural unit, the less likely liquid leakage occurs; however, when the content decreases as in Comparative Example 1-6, liquid leakage occurred. In Comparative Example 1-8, in which the mass ratio of the content of the first structural unit to the content of the second structural unit was less than 50/50, unevenness occurred on the surface of the electrolyte composition.

Test Example 2 Example 2-1 [Manufacture of Electrolyte Sheet]

A slurry containing an electrolyte composition was prepared by dispersing in an NMP, which is a dispersion medium, 21% by mass of PVDF-HFP (vinylidene fluoride/hexafluoropyrene (mass ratio) =95/5), 14% by mass of SiO₂ particles (average particle size 0.1 μm) as oxide particles, and 65% by mass of an electrolyte solution in which lithium bis(trifluoromethanesulfonyl)imide (Li[TF SI]) as an electrolyte salt was dissolved in an ionic liquid (N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-bis(trifluoromet hanesulfonyl)imide (DEME-TFSI) to a concentration of 1.5 mol/L. A polyethylene terephthalate substrate was coated with the obtained slurry, and heated to volatilize the dispersion medium to obtain an electrolyte sheet. The thickness of the electrolyte layer in the obtained electrolyte sheet was 25±2 μm. Hereinafter, the composition of the ionic liquid in which the electrolyte salt was dissolved may be expressed as “concentration of lithium salt/type of lithium salt/type of ionic liquid”.

[Manufacture of Lithium Ion Secondary Battery]

A positive electrode mixture was prepared by mixing 78.5 parts by mass of a layered lithium/nickel/manganese/cobalt composite oxide (positive electrode active material), 5 parts by mass of acetylene black (conductive agent, trade name: HS-100, average particle size 48 nm, manufactured by Denka Company Ltd.), 2.5 parts by mass of a copolymer solution of vinylidene fluoride and hexafluoropropylene (solid content of 12% by mass), and 14 parts by mass of an ionic liquid (1.5 mol/L/Li[FSI]/Py13-FSI) in which an electrolyte salt was dissolved. A positive electrode current collector (20 μm-thick aluminum foil) was coated with this positive electrode mixture slurry in a coating mass of 125 g/m², heated at 80° C., and dried to form a positive electrode mixture layer having a mixture density of 2.7 g/cm³. This was cut into a width of 30 mm and a length of 45 mm to form a positive electrode plate, and a positive electrode current collecting tab was attached to the positive electrode plate as illustrated in FIG. 2.

A negative electrode mixture slurry was prepared by mixing 78 parts by mass of a graphite 1 (negative electrode active material, manufactured by Hitachi Chemical Company Ltd.), 2.4 parts by mass of a graphite 2 (manufactured by Nippon Graphite Industries Co., Ltd.), 0.6 parts by mass of carbon fiber (conductive agent, trade name: VGCF-H, manufactured by Showa Denko K. K.), 5 parts by mass of a copolymer solution of vinylidene fluoride and hexafluoropropylene (solid content: 12% by mass), and 14 parts by mass of an ionic liquid in which an electrolyte salt was dissolved (1.5 M/Li[FSI]/Py13-FSI).

A current collector (copper foil having a thickness of 10 μm) was coated with this negative electrode mixture slurry in a coating mass of 60 g/m², heated at 80° C., and dried to form a negative electrode mixture layer having a mixture density of 1.8 g/cm³. This was cut to a width of 31 mm and a length of 46 mm to form a negative electrode plate, and a negative electrode current collecting tab was attached to the negative electrode plate as illustrated in FIG. 2.

The substrate was peeled from the electrolyte sheet and disposed between the produced positive electrode plate and negative electrode plate to produce a laminated electrode group.

The electrode group was housed in a battery outer casing made of an aluminum laminate film, as illustrated in FIG. 1. Inside the battery outer casing, the positive electrode current collecting tab and the negative electrode current collecting tab were pulled out and the opening of the battery container was sealed to manufacture the lithium ion secondary battery of Example 2-1. The aluminum laminate film was a laminate of polyethylene terephthalate (PET) film/aluminum foil/sealant layer (polypropylene, etc.). The design capacity of the manufactured lithium ion secondary battery was 20 mAh.

Examples 2-2 to 2-3

Lithium ion secondary batterys were manufactured in the same manner as in Example 2-1, except that the mass ratio of the electrolyte (electrolyte salt and solvent) content to the electrolyte support material (material other than the electrolyte salt and solvent) content (electrolyte content/electrolyte support material content) in the electrolyte sheet was changed to the mass ratio shown in Table 6.

Examples 2-4 to 2-5

Lithium ion secondary batterys were manufactured in the same manner as in Example 2-1, except that the concentration of the electrolyte salt of the electrolyte in the electrolyte sheet was changed to that shown in Table 6.

Examples 2-6 to 2-7

Lithium ion secondary batterys were manufactured in the same manner as in Example 2-1, except that the electrolyte salt and the solvent of the electrolyte in the electrolyte sheet were changed to those shown in Table 6.

Example 2-8

A lithium ion secondary battery was manufactured in the same manner as in Example 2-1, except that the average particle size of the oxide particles in the electrolyte sheet was changed to that shown in Table 6.

Examples 2-9 to 2-10

Lithium ion secondary batterys were manufactured in the same manner as in Example 2-1, except that the mass ratio of the polymer content to the oxide particle content (polymer content/oxide particle content) in the electrolyte sheet was changed to that shown in Table 6.

Examples 2-11 to 2-12

Lithium ion secondary batterys were manufactured in the same manner as in Example 2-1, except that the mass ratio of the structural units in the polymer in the electrolyte sheet was changed to that shown in Table 7.

Example 2-13

A lithium ion secondary battery was manufactured in the same manner as in Example 2-1, except that the average particle size of oxide particles, the type of electrolyte salt, the type of solvent, and the electrolyte content/electrolyte support material content in the electrolyte sheet were changed to those shown in Table 7.

Example 2-14

A lithium ion secondary battery was manufactured in the same manner as in Example 2-13, except that the average particle size of the oxide particles in the electrolyte sheet was changed to that shown in Table 7.

Example 2-15

A lithium ion secondary battery was manufactured in the same manner as in Example 2-13, except that the electrolyte content/electrolyte support material content (mass ratio) in the electrolyte sheet was changed to that shown in Table 7.

Examples 2-16 to 2-17

Lithium ion secondary batterys were manufactured in the same manner as in Example 2-13, except that the polymer content/oxide particle content (mass ratio) and the electrolyte content/electrolyte support material content (mass ratio) in the electrolyte sheet were changed to those shown in Table 7.

Example 2-18

A lithium ion secondary battery was manufactured in the same manner as in Example 2-13, except that the type of the ionic liquid for the electrolyte sheet was changed to that shown in Table 7.

Example 2-19

A lithium ion secondary battery was manufactured in the same manner as in Example 2-18, except that the average particle size of the oxide particles in the electrolyte sheet was changed to that shown in Table 7.

Example 2-20

A lithium ion secondary battery was manufactured in the same manner as in Example 2-18, except that the electrolyte content/electrolyte support material content in the electrolyte sheet was changed to that shown in Table 7.

TABLE 6 Electrolyte Electrolyte Support Material Electrolyte Content/Electrolyte Polymer Type Oxide Particles Polymer Content/Oxide Electrolyte Salt Support Material (mass ratio of (average particle Particle Content Electrolyte Concentration Content structural units) size μm) (mass ratio) Salt Type (mol/L) Solvent Type (mass ratio) Example 2-1 PVDF-HFP 0.1 3/2 Li [TFSI] 1.5 DEME-TFSI 35/65 (95/5) Example 2-2 PVDF-HFP 0.1 3/2 Li [TFSI] 1.5 DEME-TFSI 42.5/57.5 (95/5) Example 2-3 PVDF-HFP 0.1 3/2 Li [TFSI] 1.5 DEME-TFSI 50/50 (95/5) Example 2-4 PVDF-HFP 0.1 3/2 Li [TFSI] 2 DEME-TFSI 35/65 (95/5) Example 2-5 PVDF-HFP 0.1 3/2 Li [TFSI] 1 DEME-TFSI 35/65 (95/5) Example 2-6 PVDF-HFP 0.1 3/2 Li [FSI] 1.5 EMI-FSI 35/65 (95/5) Example 2-7 PVDF-HFP 0.1 3/2 Li [FSI] 1.5 Py13-FSI 35/65 (95/5) Example 2-8 PVDF-HFP 1 3/2 Li [TFSI] 1.5 DEME-TFSI 35/65 (95/5) Example 2-9 PVDF-HFP 0.1 2/1 Li [TFSI] 1.5 DEME-TFSI 35/65 (95/5) Example 2-10 PVDF-HFP 0.1 1/1 Li [TFSI] 1.5 DEME-TFSI 35/65 (95/5)

TABLE 7 Electrolyte Electrolyte Support Material Electrolyte Content/Electrolyte Polymer Type Oxide Particles Polymer Content/Oxide Electrolyte Salt Support Material (mass ratio of (average particle Particle Content Electrolyte Concentration Content structural units) size μm) (mass ratio) Salt Type (mol/L) Solvent Type (mass ratio) Example 2-11 PVDF-HFP 0.1 3/2 Li [TFSI] 1.5 DEME-TFSI 35/65 (97/3) Example 2-12 PVDF-HFP 0.1 3/2 Li [TFSI] 1.5 DEME-TFSI 35/65 (93/7) Example 2-13 PVDF-HFP 0.04 3/2 Li [FSI] 1.5 Py13-FSI 57/43 (95/5) Example 2-14 PVDF-HFP 0.01 3/2 Li [FSI] 1.5 Py13-FSI 57/43 (95/5) Example 2-15 PVDF-HFP 0.04 3/2 Li [FSI] 1.5 Py13-FSI 50/50 (95/5) Example 2-16 PVDF-HFP 0.04 3/1 Li [FSI] 1.5 Py13-FSI 51/49 (95/5) Example 2-17 PVDF-HFP 0.04 1/1 Li [FSI] 1.5 Py13-FSI 62/38 (95/5) Example 2-18 PVDF-HFP 0.04 3/2 Li [FSI] 1.5 EMI-FSI 57/43 (95/5) Example 2-19 PVDF-HFP 0.01 3/2 Li [FSI] 1.5 EMI-FSI 57/43 (95/5) Example 2-20 PVDF-HFP 0.04 3/2 Li [FSI] 1.5 EMI-FSI 50/50 (95/5)

[Evaluation of Electrolyte Sheet]

The electrolyte sheets manufactured in Examples 2-1 to 2-20 were evaluated regarding their conductivity, slurry properties, the surface properties of the electrolyte composition, and the presence or absence of liquid leakage in the same manner as in Test Example 1. The results are shown in Table 8.

[Evaluation of Initial Characteristics of Lithium Ion Secondary Batterys]

Using a charge/discharge device (BATTERY TEST UNIT, manufactured by IEM Co., Ltd.), the lithium ion secondary batterys manufactured in the above Examples 2-1 to 2-20 were charged at a constant current with a current value of 0.2 C to a charge end voltage of 4.2 V at 25° C. After resting for 15 minutes, constant current discharge was performed at a current value of 0.2 C to a discharge end voltage of 2.7 V. Charging/discharging was repeated 3 times under the above charge/discharge conditions, and the discharge capacity at the third cycle (initial capacity) was measured. The initial characteristics of the lithium ion secondary battery were calculated from the following formula (9). The results are shown in Table 8.

Initial characteristics (%)=(initial capacity/design capacity)×100   (9)

TABLE 8 Presence/ Presence/ Presence/ Initial Conductivity Absence of Absence of Absence of Characteristics (mS/cm) Slurry Gelation Unevenness Liquid Leakage (%) Example 2-1 0.21 − − − 98 Example 2-2 0.15 − − − 99 Example 2-3 0.10 − − − 98 Example 2-4 0.18 − − − 96 Example 2-5 0.25 − − − 99 Example 2-6 1.02 − − − 97 Example 2-7 0.51 − − − 97 Example 2-8 0.20 − − − 97 Example 2-9 0.15 − − − 90 Example 2-10 0.22 − − − 98 Example 2-11 0.17 − − − 96 Example 2-12 0.23 − − − 95 Example 2-13 0.48 − − − 100 Example 2-14 0.56 − − − 100 Example 2-15 0.93 − − − 100 Example 2-16 1.12 − − − 100 Example 2-17 1.84 − − − 100 Example 2-18 2.51 − − − 100 Example 2-19 2.73 − − − 100 Example 2-20 2.12 − − − 100

Test Example 3 (reference)

In order to investigate the relationship of the average primary particle size of the oxide particles included in the electrolyte composition with the tensile strength and discharge rate characteristics of the electrolyte composition formed into a sheet, the following tests were performed.

Reference Example 3-1 [Manufacture of Electrolyte Sheet]

A slurry containing an electrolyte composition was prepared by dispersing in an NMP, which is a dispersion medium, PVDF-HFP (vinylidene fluoride/hexafluoropyrene (mass ratio)=95/5), SiO₂ particles (average primary particle size of about 1 μm, trade name: AEROSIL SO-C4, manufactured by Nippon Aerosil Co., Ltd.) as oxide particles, and an electrolyte solution in which lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dried under a dry argon atmosphere was dissolved in tetraglyme (tetraethylene glycol dimethyl ether, G4) such that the electrolyte salt had a concentration of 2.3 mol/L. The electrolyte solution was further added to the slurry, followed by mixing, and the mass ratio of the polymer, oxide particles, and electrolyte solution was polymer:oxide particles:electrolyte solution=30:20:50. Thereafter, NMP was further added to adjust the viscosity, and a polyethylene terephthalate substrate (product name: Teonex R-Q51, manufactured by Teijin-DuPont Films, Inc., thickness 38 μm) was coated with the slurry using an applicator. The slurry used for coating was heated and dried at 100° C. for 2 hours to volatilize the dispersion medium and obtain an electrolyte sheet.

[Manufacture of Coin Battery Battery for Evaluation]

A coin battery battery for evaluation was manufactured using a positive electrode including LiNi_(1/3)Mn_(1/3)Co_(1/3) as a positive electrode active material, an electrolyte sheet, and a negative electrode including natural graphite as a negative electrode active material. The positive electrode, the electrolyte layer obtained by peeling the substrate from the electrolyte sheet, and the negative electrode were stacked in that order, placed in a CR2016 type coin battery container, a small amount of liquid (electrolyte solution) with the same composition as the slurry used to manufacture the electrolyte sheet was then added, and the upper part of the battery container was crimped and sealed via an insulating gasket. The electrolyte solution was added such that it was 130 to 200% of the void volume of the positive electrode and the negative electrode. The void volume of the electrode mixture layer was calculated according to the following formulae after measuring the apparent volume (area×thickness) of the electrode mixture layer excluding the current collector.

Void volume of positive electrode mixture layer=apparent volume of positive electrode mixture layer−(mass of positive electrode active material/true density of positive electrode active material)−(mass of conductive agent/true density of conductive agent)−(mass of binder/true density of binder)

Void volume of negative electrode mixture layer=apparent volume of negative electrode mixture layer−(mass of negative electrode active material/true density of negative electrode active material)−(mass of conductive agent/true density of conductive agent)−(mass of binder/true density of binder)

Reference Example 3-2

A coin battery battery for evaluation was manufactured in the same manner as in Example 3-1, except that the oxide particles used in the electrolyte sheet were SiO₂ particles (average primary particle size of about 1μm, product name: AEROSIL SX-C7, manufactured by Nippon Aerosil Co., Ltd.) that had been surface treated (phenylaminosilane treatment).

Reference Example 3-3

A coin battery battery for evaluation was manufactured in the same manner as in Example 3-1, except that the oxide particles used in the electrolyte sheet were SiO₂ particles having a different average primary particle size (average primary particle size of about 2μm, product name: AEROSIL SO-C6, manufactured by Nippon Aerosil Co., Ltd.).

Reference Example 3-4

A coin battery battery for evaluation was manufactured in the same manner as in Example 3-1, except that the oxide particles used in the electrolyte sheet were SiO₂ particles having a different average primary particle size (average primary particle size of about 0.04 μm, product name: AEROSIL OX-10, manufactured by Nippon Aerosil Co., Ltd.).

[Evaluation of Electrolyte Sheet]

(Tensile Strength)

The obtained electrolyte sheets were cut to a width of 5 mm, sandwiched between chucks, and then fixed to a pedestal with tape such that their length was 20 mm. Then, the electrolyte sheets were pulled by using a force gauge (manufactured by NIDEC-SHIMPO CORPORATION, FGP-5), and the strength when the electrolyte sheet fractured was measured. The results are shown in Table 9.

[Evaluation of Discharge Rate Characteristics of Coin Battery Battery for Evaluation]

The discharge rate characteristics of the obtained coin battery batteries at 25° C. was measured under the following charge/discharge conditions using a charge/discharge device (manufactured by Toyo System Co., Ltd.).

-   (1) A cycle in which charging was performed at a constant current     and constant voltage (CCCV) at 0.05 C to an end voltage of 4.2 V and     then discharging was performed at a constant current (CC) at 0.05 C     to an end voltage of 2.7 V was repeated twice. Here, “C” means     “current value (A)/battery capacity (Ah)”. -   (2) Next, a cycle in which charging was performed at a constant     current and constant voltage (CCCV) at 0.05 C to an end voltage of     4.2 V and then discharging was performed at a constant current (CC)     at 0.1 C to an end voltage of 2.7 V was carried out once. Further,     the rate of constant current (CC) discharge was changed to 0.2, 0.3,     0.5, and 1.0 C for each cycle, and the discharge rate     characteristics were evaluated. Table 9 shows the discharge rate     characteristics of 0.2 C with respect to 0.05 C.

TABLE 9 Average Primary Oxide Discharge Particle Size of Particles Tensile Rate Charac- Oxide Particles Surface Strength teristics (μm) Treatment (MPa) (%) Reference 1 — 2.4 92.2 Example 3-1 Reference 1 phenyl- 3.4 92.3 Example 3-2 aminosilane treatment Reference 2 — 2.7 93.2 Example 3-3 Reference 0.04 — 1.7 93.4 Example 3-4

REFERENCE SIGNS LIST

1 . . . secondary battery, 6 . . . positive electrode, 7 . . . electrolyte layer, 8 . . . negative electrode, 13A, 13B . . . electrolyte sheet, 14 . . . substrate. 

1. An electrolyte composition, comprising: one or two or more polymers; oxide particles; at least one electrolyte salt selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt; and a solvent, wherein structural units constituting the one or two or more polymers comprise a first structural unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride, and a second structural unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate, a content of the one or two or more polymers is more than 90% by mass based on a total amount of polymers in the electrolyte composition, and a mass ratio of a content of the first structural unit to a content of the second structural unit in the one or two or more polymers is 50/50 or more.
 2. The electrolyte composition according to claim 1, wherein the one or two or more polymers comprise a copolymer comprising both the first structural unit and the second structural unit.
 3. The electrolyte composition according to claim 1, wherein the one or two or more polymers comprise at least two polymers of a first polymer comprising the first structural unit and a second polymer comprising the second structural unit.
 4. The electrolyte composition according to claim 1, wherein the content of the one or two or more polymers is 3 to 50% by mass based on a total amount of the electrolyte composition.
 5. The electrolyte composition according to claim 1, wherein the oxide particles are particles of at least one selected from the group consisting of SiO2, Al2O3, AOOOH, MgO, CaO, ZrO2, TiO2, Li7La3Zr2O12, and BaTiO3.
 6. The electrolyte composition according to claim 1, wherein a content of the oxide particles is 5 to 40% by mass based on a total amount of the electrolyte composition.
 7. The electrolyte composition according to claim 1, wherein the solvent is a glyme represented by the following formula (1), R1O—(CH2CH2O)k—R2   (1) wherein R1 and R2 each independently represent an alkyl group having 4 or less carbon atoms or a fluoroalkyl group having 4 or less carbon atoms, and k represents an integer of 1 to
 6. 8. The electrolyte composition according to claim 1, wherein the solvent is an ionic liquid.
 9. The electrolyte composition according to claim 8, wherein the ionic liquid comprises, as a cation component, at least one selected from the group consisting of a chain-like quaternary onium cation, a piperidinium cation, a pyrrolidinium cation, a pyridinium cation, and an imidazolium cation, and the ionic liquid comprises, as an anion component, at least one of anion components represented by the following formula (2), N(SO2CmF2m+1)(SO2CnF2n+1)-   (2) wherein m and n each independently represent an integer of 0 to
 5. 10. The electrolyte composition according to claim 1, wherein a total content of the electrolyte salt and the solvent is 25 to 70% by mass based on a total amount of the electrolyte composition.
 11. The electrolyte composition according to claim 1, formed into a sheet.
 12. A secondary battery, comprising: a positive electrode; a negative electrode; and an electrolyte layer consisting of the electrolyte composition according to claim 1, disposed between the positive electrode and the negative electrode.
 13. A method for manufacturing an electrolyte sheet, comprising: disposing on a substrate a slurry comprising one or two or more polymers, oxide particles, at least one electrolyte salt selected from the group consisting of a lithium salt, a sodium salt, a calcium salt, and a magnesium salt, a solvent, and a dispersion medium; and volatilizing the dispersion medium to form an electrolyte layer on the substrate, wherein structural units constituting the one or two or more polymers comprise a first structural unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride, and a second structural unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate, a content of the one or two or more polymers is more than 90% by mass based on a total amount of polymers in a solid content of the slurry, and a mass ratio of a content of the first structural unit to a content of the second structural unit in the one or two or more polymers is 50/50 or more. 